Tumors that differ from one another in immunogenicity will be employed to document age-determined changes in immune resistance to tumor growth and dissemination. Mice ranging in age from 1 1/2 months to 25 months will be studied, with the expectation that an age-determined increase in antitumor resistance will precede a precipitous age-determined decline, as determined by the time of survival after intradermal implantation of tumor cells. The immunogenicity (ability to immunize against an implant) of the tumors will be measured by a standard procedure, and the information obtained will be used to test the prediction that the largest age-determined changes in anti-tumor resistance will be seen with tumors of higher immunogenicity, because there will be a greater degree of immune responsiveness to undergo decline. On the other hand, young mice and aged mice should not differ in their ability to survive implantation with non- immunogenic tumors. The above experiments will also be performed in T cell-deficient mice to show that the differences in tumor growth and in host survival are T cell dependent. Direct measurements of the effect of aging on the underlying concomitant anti-tumor immunity responsible for the predicted differences in survival times will be made by employing limiting- dilution frequency analysis to enumerate cytolytic T cells (CTL) and their precursors (CTL-P) in draining lymph nodes and spleens against time of tumor growth. These experiments will be supplemented by those that will enumerate CTL-P production in mice of different ages bearing a tumor injected with C. parvum to augment CTL-P production and to cause tumor regression in young adult animals. It is predicted that in contrast to its therapeutic action in young mice, C. parvum will not cause tumor rejection, or result in increased survival times in old mice, and that this will be associated with the production of a smaller number of CTL-P.